flang: check CI instead of GITHUB_ACTIONS env var.
flang: check CI instead of GITHUB_ACTIONS env var.

autobuild filters out all env vars starting with GITHUB_ or RUNNER_, so
this check failed there.

What I really want to detect is whether it is running on a Github-hosted
runner, but I don't know a good way to do so.

Alexpux/MINGW-packages,oscarfv/MINGW-packages,stahta01/MINGW-packages,oscarfv/MINGW-packages,JPeterMugaas/MINGW-packages,fabiangreffrath/MINGW-packages,oroppas/MINGW-packages,JPeterMugaas/MINGW-packages,stahta01/MINGW-packages,fabiangreffrath/MINGW-packages,oroppas/MINGW-packages,stahta01/MINGW-packages,oroppas/MINGW-packages,stahta01/MINGW-packages,Chocobo1/MINGW-packages,lhmouse/MINGW-packages,JPeterMugaas/MINGW-packages,fabiangreffrath/MINGW-packages,stahta01/MINGW-packages,oroppas/MINGW-packages,oscarfv/MINGW-packages,fabiangreffrath/MINGW-packages,Alexpux/MINGW-packages,lhmouse/MINGW-packages,Alexpux/MINGW-packages,Alexpux/MINGW-packages,oscarfv/MINGW-packages,JPeterMugaas/MINGW-packages,InitialForce/MINGW-packages,oroppas/MINGW-packages,JPeterMugaas/MINGW-packages,InitialForce/MINGW-packages,InitialForce/MINGW-packages,oroppas/MINGW-packages,stahta01/MINGW-packages,lhmouse/MINGW-packages,Alexpux/MINGW-packages,lhmouse/MINGW-packages,Alexpux/MINGW-packages,oscarfv/MINGW-packages,fabiangreffrath/MINGW-packages,fabiangreffrath/MINGW-packages,Chocobo1/MINGW-packages,JPeterMugaas/MINGW-packages,lhmouse/MINGW-packages,Chocobo1/MINGW-packages,oscarfv/MINGW-packages,lhmouse/MINGW-packages